This invention relates to an apparatus and a method for coating diamond like carbon (DLC) or other vacuum depositable material onto a substrate.
DLC films or coatings is the generic term for a mixture of sp2 and sp3 bonded carbon and can generally be described as hard, amorphous, lubricious, impermeable, chemically inert and possessing high electrical resistivity proportions.
Presently medical device coatings such as polytetrafluoroethylene (PTFE) or Silicone overlays are applied by dip coating or electrostatic spraying methods thus forming a 200-300 xcexcm thick outer layer. Traditionally these coating techniques have associated problems including poor metal to polymer adhesion and non uniformity of coating. The nature of these coating techniques leads to poor process control due to the fluid dynamics of thick films, poor yields due to adhesion and poor uniformity and large non-environmentally friendly waste.
The most commonly used technique for coating guide wires is electrostatic spraying which gives rise to non-uniform coatings and requires the need for interlayers between the substrate and coating to achieve acceptable adhesion. Additionally excessively thick coatings subsequently adds rigidity to the substrate which may be, for example, a wire or a spring. Furthermore the process gives rise to loosely bonded aspirates which are not acceptable where the substrate is designed for internal use such as medical devices for body implantation.
Thus, for example, medical grade stainless steel guide wires are currently used to aid the introduction of catheters and other medical devices into the human body. The device in a coated or uncoated state must exhibit good flexibility, low surface roughness, possess a high chemical resistance and conform to biocompatibility standards. The device requires surface hardness as it has to be threaded through a metal needle; high flexibility and low surface coefficient of friction in order to aid the movement through channels in the body; and good biocompatibility properties due to the nature of the in vivo procedure.
In the prior art, and with reference to FIG. 1 of the drawings, plasma based coating systems, such as sputtering, and normal RF plasma enhanced chemical vapour deposition chambers, employ the use of parallel plate or conical facing electrodes that only allow xe2x80x98in the plane of sightxe2x80x99 coating. Thus rotation of the substrate is required. Thus in the case of, for example, sputtering, in FIG. 1, the substrate 1 is interposed between two plates 2, 3 in a chamber having walls 5. The plate 2 acts as a cathode; the plate 3 acts as an anode or the walls 5 act as an anode. A source of radio frequency voltage is provided from a source 4. Apart from the need to rotate the substrate 1 relative to the plates 2,3, the coating of the substrate is uneven, non-uniform and non-conformal. This is particularly the case where the surface of the substrate is, at the microscopic level, uneven.
It is an object of the present invention to provide an apparatus for applying a conformal DLC coating, or other vacuum depositable material, to a substrate.
The invention, therefore, provides an apparatus for coating a substrate with a diamond like coating or other vacuum depositable material, the apparatus comprising a chamber having an anode, means for supporting a substrate in the chamber, means for establishing a low pressure gaseous atmosphere in the chamber, and a radio frequency source for establishing a gas plasma in the chamber, the substrate acting as a cathode.
The invention further provides a method for coating a substrate with a diamond like coating or other vacuum depositable material, the method comprising supporting a substrate in a chamber having an anode, establishing a low pressure gaseous atmosphere in the chamber, and establishing a gas plasma in the chamber using a radio frequency source, the substrate acting as a cathode.
By providing an apparatus in which the substrate is the cathode and the wall or preferably walls of the chamber function as the anode, the substrate may remain static, i.e. rotation of the substrate is not required in order to uniformly coat the substrate. This is particularly advantageous where the substrate comprises, in the microscopic sense, a rough surface. Alternatively, the anode may comprise a plate inside the chamber or may comprise a tubular element in the chamber and into which element the substrate is placed. Instead of using a tubular element, a pair of oppositely disposed hemispherically shaped elements could act as an anode. As a further alternative, the chamber itself could be tubularly shaped in which the curved wall is an anode. In use, a coaxial plasma can form between the cathode and the anode electrode arrangements that are spaced typically over ranges of between 2 cm and 30 cm although this gap can be larger or smaller. The geometry of the anode electrode may be circular or square but in the case of a short gap of the order of a few cms it is preferred, for a conformal coating, that the anode is circular. In the case of a wide gap i.e. greater than 10 cm the geometry of the anode is less important and a square/rectangular arrangement can be utilised due to the nature of electric field confinement over larger distances on small diameter substrates.